This invention relates to a sootblower cleaning apparatus employed to direct jets of air, steam, water, or a mixture of such agents against fouled or slag encrusted components of large scale boilers and other heat exchangers used by public utilities or by industry for the production of steam for power generation and other purposes. The invention relates particularly to sootblowers of the long retracting type, which are moved into the boiler to clean and then withdrawn from the severe environment therein. Sootblowers of this type employ a long retracting lance typically having two or more radially directed nozzles near the tip of the lance.
Typically, as a long retracting sootblower lance is inserted into and retracted from the boiler, it is simultaneously rotated and/or oscillated about its longitudinal axis so that the blowing medium jet emitted from the nozzles sweeps a helical or partially helical path. The lance typically rotates a number of times during its projection and retraction movement. The relationship between the translational and rotational movement of the lance tube determines the helix distance, i.e. the longitudinal distance between helical sweeps of the lance nozzle jet. Helix distance is dictated by the cleaning requirements for a particular application. Cleaning requirements also determine the speed at which the helical jet is advanced. The speed at which the lance may safely be rotated must be maintained below a critical speed at which the lance becomes dynamically unstable. Therefore, the minimum total cycle time required to insert and retract the lance becomes limited by this consideration. In applications where cleaning requirements do not control the rate of helical advancement of the blowing medium jet, the cycle time of the sootblower is dictated solely by the critical speed characteristics. In such instances, a certain minimum flow of blowing medium must be maintained through the lance in order to provide sufficient cooling for the lance to protect it in the severe environment within the boiler, resulting in a considerable waste of blowing medium. Moreover, longer than necessary sootblower cycle time leads to increased power consumption and unnecessary component wear.
This invention is directed to optimizing the cycle duration of a long retracting type sootblower for applications wherein the cycle time during a part of or the entire operating cycle is primarily dictated by the dynamic instability of the lance tube.
Dynamic instability results when the rate of rotation of the lance tube, which is supported by a traveling carriage and by a support near the boiler wall, exceeds the critical speed which is characteristic of the particular sootblower configuration. Dynamic instability results in a resonance condition which can have a highly destructive effect on the lance tube and associated mechanisms. The critical speed at which dynamic instability occurs is a function of the sootblower type and configuration, and occurs at a lower speed when the lance tube is fully inserted into the boiler than when the lance tube is partially inserted.
A principal aspect of this invention is to optimize the total cycle time of a sootblower apparatus of the long retracting type by controlling the speed at which the lance tube is rotated in accordance with the projected length of the lance tube within the boiler and the characteristics of the device such that the rotational speed remains below the critical speed for the lance at each projected length. Since the lance tube becomes unstable at higher speeds at intermediate projected lengths, the lance tube may be safety driven at higher speeds in those positions. By driving the sootblower lance at intermediate projected lengths at a rotational speed greater than the critical speed for a fully extended lance, shorter cycle times are achievable compared to sootblowers according to the prior art wherein constant driving speeds are used. Cycle time reductions are realized for sootblowers having a fixed ratio between the speed of rotation and translation of the lance tube since increases in rotational speed translates into increases in translational speed and therefore cycle time. In sootblower types wherein the speed of lance tube rotation and translation are independently controllable, for example those having separate drive motors, cycle time reduction may be realized since the rotational and translational speeds of the lance tube may be modulated in accordance with the extended length of the lance thereby resulting in cycle time reductions.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates upon a reading of the described preferred embodiments of this invention taken in conjunction with the acompanying drawings.